The present invention relates to a color cathode ray tube and more particularly to a color cathode ray tube having an electron gun providing a satisfactory resolution over the entire picture with a comparatively low dynamic focus voltage.
In a color cathode ray tube used as a color picture tube or a display tube, it is necessary to control the focus characteristic of the electron gun properly according to the angle of deflection of electron beams so as to provide a satisfactory resolution always over the entire screen.
FIG. 3 is a cross sectional schematic view illustrating the structure of this kind of conventional color cathode ray tube. Numeral 1 indicates an evacuated glass envelope, 2 a faceplate portion constituting a screen, 3 a phosphor screen, 4 a shadow mask, 5 an internal conductive coating, 6, 7, and 8 cathodes, 9 a first grid electrode (G1 electrode), 10 a second grid electrode (G2 electrode), 11 a third grid electrode (G3 electrode), 12 a fourth grid electrode (G4 electrode), 13 a fifth grid electrode (G5 electrode), 14 an accelerating electrode (G6 electrode), 15 a shield cup, 16 a deflection yoke, 17, 18, and 19 initial paths of electron beams, and 20 and 21 center lines of passage apertures of outer electron beams (hereinafter may be referred to as apertures) formed in the accelerating electrode 14.
In the figure, a phosphor screen 3 comprising an alternate line pattern of red, green, and blue emitting phosphors is supported on the inner wall of the faceplate portion 2 of the evacuated glass envelope 1. The center lines (the initial paths of electron beams) 17, 18, and 19 of the cathodes 6, 7, and 8 coincide with the center lines of apertures associated with corresponding cathodes, of the G1 electrode 9, the G2 electrode 10, and the G3 electrode 11, the G4 electrode 12, and the G5 electrode (focus electrode) 13, these three constituting the main lens, and the shield cup 15 and are arranged almost in parallel with each other in a common plane (inline arrangement).
The center line of the aperture at the center of the G6 electrode (accelerating electrode) 14 which is another electrode constituting the main lens coincides with the center line 18. However, the center lines 20 and 21 of both the apertures on the outer side do not coincide with the center lines 17 and 19 corresponding to them but are slightly displaced outwardly.
Three electron beams emitted from the cathodes 6, 7, and 8 enter the final lens (main lens) formed between the G5 electrode 13 and the G6 electrode 14 along the center lines 17, 18, and 19.
A focus voltage Vf of about 5 to 10 kV is applied on the G3 electrode 11 and the G5 electrode 13 and an accelerating voltage Eb which is the highest voltage of about 20 to 30 kV is applied on the G6 electrode 14 via the conductive coating 5 and the shield cup 15 placed in the evacuated glass envelope 1.
The center lines of the apertures at the centers of both of the G5 electrode 13 and the G6 electrode 14 constituting the final lens for focusing electron beams on the phosphor screen 3 are coaxial, so that a lens formed in the aperture portion at the center is axially symmetric and an electron beam (center beam) passing through the aperture at the center is focused by the final lens and goes straight along the axis.
On the other hand, the center lines of the outer apertures of both the electrodes constituting the final lens are displaced from each other, so that a non-axially-symmetric lens is formed in the outer aperture portion. As a result, an electron beam (outer beam) passing through the outer apertures passes through a portion displaced toward the center beam from the center line of the lens in the diverging lens region formed on the side of the accelerating electrode (G6 electrode) 14 in the lens region, so that it is subjected to the focusing action by the lens and the converging force toward the center beam at the same time.
Also known is a type of an electron gun in which each of two electrodes constituting a final lens has a single horizontally elongated opening at their opposing ends and has a plate electrode therein having beam passage apertures retracted inwardly from the opposing ends.
Also in this type of an electron gun, a non-axially-symmetric lens is formed in the outer aperture portion of both the electrodes and the outer electron beams are given the converging force toward the center beam, and the three electron beams are converged so as to be superposed in the plane of the shadow mask 4.
An operation for converging each electron beam by an electrode structure like this is referred to as a static convergence (STC).
Furthermore, each electron beam is subjected to color selection by the shadow mask 4 and only a portion of each electron beam passes through an aperture of the shadow mask 4 for exciting the phosphor of a color corresponding to the electron beam on the phosphor screen 3 to luminescence and reaches the phosphor screen 3.
A magnetic deflection yoke 16 for scanning electron beams on the phosphor screen 3 is mounted outside the funnel portion of the evacuated glass envelope 1.
As mentioned above, it is known that when an inline electron gun in which three electron beam passage apertures are arranged in a horizontal plane and a so-called selfconverging type deflection yoke for forming a special nonhomogeneous magnetic field distribution are combined, by adjusting a self-convergence of the three beams at the center of the picture, the convergence can be adjusted over the entire remaining picture at the same time. However, when the self-converging type deflection yoke is used, a problem arises that large aberration due to deflection are generated by non-uniformity of the magnetic field and the resolution at the corners of the screen lowers.
FIG. 4 is a schematic view illustrating beam spots on the screen by an electron beam subjected to aberrations due to deflection. Numeral 3 indicates a phosphor screen (hereinafter may be referred to as a screen) and 3a, 3b, and 3c beam spots. In the figure, the beam spot 3a is almost circular at the center of the screen 3. However, at the corners of the screen, as indicated by the beam spots 3b and 3c, a high brightness portion indicated by hatching (core) c widens in the horizontal direction (X--X direction) and a low brightness portion (halo) h widens in the vertical direction (Y--Y direction) and the resolution lowers. Conventionally, as an example for solving such a problem, an electron gun is disclosed in U.S. Pat. No. 5,212,423 (corresponding Japanese Patent Application Laid-Open Hei 4-43532).
FIG. 5 is an illustration for the constitution of an electron gun of the prior art for reducing the lowering of the resolution at the corners of the screen.
In the figure, the G5 electrode 13 is divided into four parts such as a first member 13h, a second member 13i, a third member 13j, and a fourth member 13k toward the phosphor screen from the cathode.
A single opening is provided in the end face of the third member 13j opposite to the fourth member 13k and a plate electrode 13l having an electron beam passage aperture is located therein.
Plate correction electrodes 13m are located at the end face of the fourth member 13k opposite to the third member 13j so as to sandwich the electron beam passage aperture vertically and extend into the third member 13j through the single opening of the third member.
A voltage Vd varying dynamically in synchronization with the deflection current supplied to the deflection yoke is applied on the second member 13i and the fourth member 13k and a fixed voltage Vo is applied on the first member 13h and the third member 13j.
By using such a constitution, an electrostatic quadrupole lens having a function for changing the cross sectional shape of an electron beam into a non-axially symmetrical one in accordance with the amount of deflection of the electron beam is formed between the third member 13j and the fourth member 13k. Between the two aforementioned voltages Vo and Vd, there is a relationship of Vo&gt;Vd.
The final lens (main lens) formed between the fourth member 13k and the G6 electrode 14 produces an effect for focusing an electron beam horizontally stronger than vertically.
In such a structure of an electron gun, when an amount of deflection is small, the voltage difference between the third member 13j and the fourth member 13k is large, so that a cross section of the electron beam is elongated horizontally by the electrostatic quadrupole lens but it is offset by the astigmatism of the final lens elongating the cross section of the electron beam strongly vertically and degradation of the resolution at the center of the screen is prevented.
On the other hand, when an amount of deflection is large, the voltage Vd varying dynamically in synchronization with the deflection current increases and the potential difference between the third member 13j and the fourth member 13k decreases. Therefore, the strength of the electrostatic quadrupole lens weakens and the cross sectional shape of the electron beam is vertically elongated by a function of the final lens for focusing strongly horizontally.
Namely, the astigmatism caused in the electron beam produces an effect that the core c is elongated vertically and the halo h is elongated horizontally. Therefore, the astigmatism caused by the deflection of an electron beam shown in FIG. 4 can be eliminated and the resolution at the corners of the screen can be improved.
In the color cathode ray tube, the distance from the final lens to the corners of the screen is longer than the distance to the center of the screen, so that the electron beam focusing condition, that is, the focus voltage is different between the center and the corners of the screen. When this focus voltage is fixed at the voltage at which an electron beam is focused at the center of the phosphor screen, a problem arises that an electron beam is not focused at the corners of the phosphor screen and hence the resolution lowers.
However, in the constitution example of a conventional electron gun explained in FIG. 5, when the electron beam is deflected toward the corners of the screen, the potential of the fourth member 13k is increased, so that the potential difference from the accelerating voltage Eb of the accelerating electrode 14 reduces and the strength of the final lens weakens. As a result, the electron beam focusing point moves toward the phosphor screen and the electron beam can be focused also at the corners of the phosphor screen. Namely, since the electron gun has a function for correcting the curvature of the image field, degradation of the resolution at the corners can be prevented also from this point of view.
At the same time, the strengths of both the lens formed between the first member 13h and the second member 13i constituting a part of the G5 electrode 13 and the lens formed between the second member 13i and the third member 13j constituting another part of the G5 electrode 13 weaken as the dynamically varied voltage (dynamic focus voltage) Vd increases. Namely, since the two aforementioned lenses also have a function for correcting the curvature of the image field, an efficient correction of curvature of the image field can be made. These two lenses are called a correction lens for curvature of the image field.
Namely, dynamic correction of astigmatism and correction of curvature of the image field can be realized by a comparatively low dynamic focus voltage at the same time.